1. Field of the Invention
The present invention relates to an image heating apparatus for heating an image on a recording material, and more particularly to an image heating apparatus adapted for use as a fixing apparatus in a copying apparatus or a printer.
2. Related Background Art
Conventionally the temperature control of the fixing apparatus has been executed for the target temperature only, independently from the function of the main apparatus such as the copying apparatus.
However, based on the standpoint of saving the entire power consumption in the copying apparatus, there is recently made an attempt to alter the temperature control for the fixing apparatus by a signal synchronized with the function of the main apparatus. More specifically, in case the electric power is required in another unit (for example the original illuminating lamp) in the copying apparatus while the heater of the fixing apparatus is turned on, the electric power consumption in the fixing apparatus is lowered during the corresponding period.
While a lower power consumption is desired, the restriction on flicker etc. is becoming stricter year after year. As the flicker depends on the magnitude of the electric power supplied to the load and the cycle of electric power input, the apparatus has to be designed in consideration of the balance of these factors.
In the following there will be explained, with reference to FIGS. 2, 3 and 5, the configuration of a fixing apparatus in consideration of the electric power consumption and the countermeasure against flicker, and an example of the control therefor. This apparatus is to control upper and lower two heaters based on the temperature detected by a thermistor positioned at the side of a pressure roller. The thermistor at the side of the pressure roller is provided for detecting the excessive temperature rise in a sheet non-passing area of the fixing roller.
Referring to FIG. 2, at the start of the fixing temperature control, in order to raise the fixing unit to a predetermined temperature, the heater of the fixing roller is controlled (turned on in this case) based on the temperature of the pressure (lower) roller. Then, after a predetermined time (1 second in this case), the heater of the pressure roller is controlled (turned on in this case) based on the temperature of the pressure (lower) roller.
Since the electric power supply to the two heaters is not started at the same time but is mutually displaced to reduce the electric power input at a time. Also the flicker can be made less conspicuous as the electric power is turned on at a relatively long cycle time.
FIG. 3 shows a state, under such control, when the fixing temperature control is altered by an external interruption, namely by the electric power consumption in another load.
Also in this case, at the start of fixing temperature control, the heater of the fixing roller is controlled (turned on in this case) based on the temperature of the pressure (lower) roller. After a predetermined time (1 second in this case), the heater of the pressure roller is controlled (turned on in this case) based on the temperature of the pressure (lower) roller. If an interruption signal is received in this state, the temperature control method for the fixing apparatus is altered (by alternate activation of the upper and lower heaters).
When the interruption signal is terminated thereafter, the original fixing temperature control is restored. In this case the control is started from the initial state, so that the heater of the fixing roller is at first controlled (turns on in this case) based on the temperature of the pressure (lower) roller. After a predetermined time interval (1 second in this case), the heater of the pressure roller is controlled (turned on in this case) based on the temperature of the pressure (lower) roller.
Therefore, in case the interruption signals arrive at an interval of about 3 seconds, the upper heater alone is turned on for 1 second after the termination of the interruption signal as shown in FIG. 3, so that the upper heater is turned on for an approximately doubled period. As a result, the fixing roller becomes deviated from the target temperature, resulting in drawbacks such as an offset phenomenon or a shortened service life of the fixing roller.